1. Field of the Invention
The present invention relates generally to recording apparatus for forming (recording) images using a recording head having arrays of recording elements and more particularly to a recording apparatus having a head-shading function in determining density data on each recording element of a recording head by detecting the uneven density of a pattern recorded by the recording head to control the output of each recording element according to the density data, and to a head-shading method thereof.
2. Description of the Related Art
With the spread of copying machines, word processors, information processing equipment such as computers, to say nothing of communications equipment, digital image recording apparatuses apparatus employing ink-jet or thermal transfer type recording heads have become widely used as image forming (recording) apparatus for the aforementioned equipment. In such a recording apparatuses, a recording head having arrays of recording elements integrally disposed is generally used for improving recording speed.
For example, the so-called multi-nozzle head which integrally incorporates a plurality of ejection orifices and that of liquid passages is commonly used for the ink-jet recording head. On the other hand, the thermal head such as a thermal transfer head normally has a plurality of heaters integrally arranged.
With respect to the recording heads, there arise difficulties in making uniform the properties of a plurality of recording elements due to variations in the properties thereof and those in head component materials during the process of manufacture. As a result, the recording elements of such recording heads are naturally subject to variations in properties to some degree. For example, ejection orifices and liquid passages of ink-jet recording heads tend to develop variations in shape, whereas heaters of thermal heads are not also free from variations in shape and resistance. In addition to the reasons stated above in view of the limitation of production technology, secular change may also cause element-to-element variations in properties. The lack of uniformity of properties among the recording elements of the recording head results in the unevenness of the size and density of the dot recorded by each recording element and consequently the unevenness of the recorded image.
Such variations in the properties of the recording elements of the recording head (equivalent to unevenness in the amount of ink ejection in the case of an ink-jet recording head, for example) considerably deteriorate recorded image quality. Consequently, attempts have heretofore been made to compensate for the variations in the properties thereof.
A recording apparatus having the following construction has been proposed to implement such attempts. More specifically, the recording apparatus is provided with a recording pattern read unit so that, by periodically reading uneven density in the range of recording element arrays, head-shading data is prepared from the uneven density data.
By taking the ink-jet recording apparatus as an example, the head-shading method will subsequently be described. The recording head of this recording apparatus is designed to eject ink droplets under bubble generating pressure by forming ink bubbles in a plurality of ejection orifices while causing electrothermal transducers fitted in the respective ejection orifices to generate heat. The head is so constituted that it can scan a range corresponding to the length (297 mm) of a short side of a recording medium of A3 size, and is provided with arrays of ejection orifices at a density of 400 dpi (dot per inch), normally 128 of ejection orifices are arranged in a row along the direction met at right angle to the scanning direction of the head. In the case of recording in color, four of the heads thus constructed are employed, these being cyan, magenta, yellow and black heads.
In order to correct the unevenness of ink ejection (unevenness in ink density) from each ejection orifice of the recording head, it has been made a condition that each ink ejection orifice should properly correspond to the recording density data read by a read system.
In the prior art, a predetermined uniform recording signal is used to drive each ejection orifice of a recording head to form a test pattern 2 for detecting uneven density on a recording medium 1 as shown in FIG. 1. The test pattern 2 is formed on a color basis, for example. The test pattern 2 is formed in such a way that, as shown on the left-hand side of FIG. 2, three of upper, middle and lower lines 2a, 2b, 2c are printed from left to right by the head having arrays of ejection orifices arranged in rows. The method of forming the pattern 2 is called irregular 3-line printing. With 128 ejection orifices, for example, the first line 2a is printed by ejecting ink from the 96th up to the last 128th ejection orifices to start with. Subsequently, the second line 2b is printed by ejecting ink from the 1st up to the 128th ejection orifices, that is, all the ejection orifices. The third and last line 2c is printed by ejecting ink from the 1st up to the 32nd ejection orifices.
Thus the test pattern 2 is conventionally formed by sandwiching the second line 2b printed by driving all the ejection orifices of the head between the first and second lines 2a, 2c printed by driving the plurality of ejection orifices in the respective end portions thereof. If the test pattern is formed with only the second line 2b, both ends of the density data for reading will not indicate a clear rise in density because of the light reflected from blank portions near the respect ends of the pattern, thus making it difficult to settle the end positions of the head from the density data. In order to remedy the drawback, the test pattern is formed by the irregular 3-line printing.
An image read system is then used to read the test pattern 2 of certain color thus formed in direction of arrow Y from a read starting position S up to a read terminating position F as shown on the left-hand side of FIG. 2 and density distribution data thus read are temporarily stored in a memory of the apparatus in the order in which the data are read.
Incidentally, the recording density in the ink-jet recording system and the read resolution in the image read system have been set identical, for example, at 400 dpi (dot per inch) in this conventional recording apparatus. For this reason, the ink dot ejected from each ejection orifice corresponds to one pixel in the read system. If the density data stored in the memory is to be expressed by 256 gradations, the printing intensity (density) may be expressed by means of one ejection orifice corresponding to one byte area over the memory. This is because one byte consists of binary 8 bits as is well known and because the number of their combinations comes up to 2.sup.8 =256. Therefore, a section (the number of bytes) of density data well over the threshold level DTH conforms to an ejection section in the direction in which the test pattern is read on condition that the threshold level is properly set.
A section X.sub.1 to X.sub.2 in a graph on the right-hand side of FIG. 2 represents the section of the aforesaid test pattern. As X.sub.1 and X.sub.2 are obtainable as address data over the memory, it is possible to obtain addresses at which the density data at the 1st up to 128th ejection orifices are stored by computing the addresses. As a result, the density data may be used to operate on the amount of head-shading.
In the example as noted previously, however, the threshold level (D.sub.TH) will have to be properly selected to obtain X.sub.1 and X.sub.2 of FIG. 2. Particularly in the case of the test pattern printed with yellow ink, the density of the color read by the read system is lower than that of any other one and this allows the section of the test pattern to be detected as a section X.sub.3 to X.sub.4 of FIG. 2 (when the threshold level is at D.sub.TH1), depending on the way of determining the threshold level (D.sub.TH). Therefore, the conventional recording apparatus has a drawback in that the ink ejection orifice and the density data will not properly match.